Shibashis Halder

A quinoline based Schiff-base compound as pH sensor

A new Schiff-base compound, 1,4-bis-(quinolin-6-ylimino methyl)benzene (BQB), has been synthesized by the reaction between terephthaldehyde and 6-aminoquinoline (1 : 2 ratio) and characterized by elemental analysis and different spectroscopic methods. The Schiff-base compound, designated as BQB, undergoes protonation in acidic medium and has been designated as PBQB. PBQB shows emission intensity at 550 nm at low pH and with the increase in pH, the intensity of the peak at 550 nm decreases with simultaneous shift of the emission band to 453 nm. The intensity of the peak at 453 nm increases as pH of the medium is raised and the phenomenon is found to be reversible. Reversibility of fluorescence intensity of BQB with pH shows that the system is stable in both basic and acidic media. Absorption spectrum of the compound shows band at 380 nm with a strong shoulder at 402 nm in low pH which changes to 344 nm at higher pH region. Naked eye detection of colour change at different pH regions is possible using the probe. Position of absorption band of BQB is changed in acidic medium due to protonation of the ring nitrogen atom present in it as reflected by theoretical studies. We have applied our probe to detect the pH region of river water.

Chromogenic and fluorescence sensing of pH with a Schiff-base molecule

pH is one of the key parameters of many chemical and biological processes and also to assess the quality of water. We have synthesized and characterized a new Schiff-base molecule, 2-hydroxy-5-methyl-3-((quinolin-3-yliminio)methyl)benzaldehyde (HMQB) which is able to detect different pH regions. It shows a strong emission band at 464 nm in Britton Robinson buffer solution of pH 2.0 upon excitation at 400 nm. With the increase in pH value of the medium, the emission intensity at 464 nm decreases gradually and at the same time, a new fluorescence peak emerges at 530 nm. The UV-vis spectra of HMQB exhibits peaks at 350 nm and 435 nm in acidic and basic regions respectively. Images of HMQB in different pH solutions indicate no coloration and yellowish green coloration in acidic and basic media respectively corroborating the results of the UV-vis spectra. We have applied the probe molecule successfully to sense pH regions of river water and lemon juice. HMQB has also been used for cell imaging study.

Mono-, tri- and polynuclear copper(II) complexes of Schiff-base ligands: synthesis, characterization and catalytic activity towards alcohol oxidation

Three new copper(II) complexes with different Schiff-base ligands, namely, [Cu(L1)Cl2] (1), [Cu3(L2)2Cl4] (2) and [Cu(L3)N3]n (3) where L1 = 2-morpholino-N-(pyridin-2-ylmethylene)ethanamine, HL2 = 4-bromo-2-((2-morpholinoethylimino)methyl)phenol and HL3 = 4-chloro-2-((2-(dimethylamino)ethylimino)methyl)phenol, have been synthesized and characterized by elemental analysis, standard spectroscopic methods, cyclic voltammometry and single crystal X-ray diffraction analysis. The X-ray diffraction analysis confirms the formation of mononuclear (1), trinuclear (2) and polynuclear (3) complexes. These complexes have been applied as catalysts for alcohol oxidation reactions using tert-butyl hydroperoxide (TBHP) as the terminal oxidant under mild conditions. The catalytic reaction mixture has been analyzed by gas chromatography and it shows that the mononuclear complex has the highest conversion while the other two complexes exhibit moderate catalytic activities. The corresponding aldehyde has been obtained as the sole product. The obtained catalysis results have been corroborated with electrochemical studies.

A quinoline-based compound for explosive 2,4,6-trinitrophenol sensing: experimental and DFT-D3 studies

A quinoline-based Schiff base compound, 2,5-dimethylbis(quinolin-2-ylmethylene)benzene-1,4-diamine (DQB), has been found to be a chemosensor for 2,4,6-trinitrophenol (TNP). DQB has been synthesized by condensation between two equivalents of quinoline-4-carboxaldehyde and one equivalent of 2,5-dimethyl-1,4-diaminobenzene in methanol under ambient reaction conditions and characterized by using standard spectroscopic methods. It shows an emission band at 440 nm with high intensity (λex = 390 nm). The emission intensity is quenched severely in the presence of TNP while other nitroaromatic compounds fail to reduce the intensity, signifying high selectivity towards TNP. The fluorescence quenching mechanism can be explained by theoretical calculations as an RET-ICT based pathway.